Objects of high-strength aluminium are often used as structural materials for machine parts on which demands are placed for light weight and high strength, for example in aircraft structures.
High-strength aluminium is obtained by precipitation hardening (or so-called age hardening) of a so-called heat treatable aluminium alloy by a two-stage heat treatment process. In the first stage, during the so-called precipitation treatment, the material is heated to an elevated temperature at which all alloy components are dissolved in the crystal lattice structure of the aluminium and are transformed into so-called solid solution. The greater the proportion of alloy components which the alloy contains, the higher will be the temperature required for solution. The solution treatment is terminated in that the object is rapidly cooled with water, water mist or air. In the second stage, during the so-called ageing process, hardening precipitations are formed in the material. Ageing of high-strength aluminium takes place at elevated temperature for a relatively short time, so-called artificial ageing, as opposed to so-called cold ageing, i.e. ageing at room temperature over a relatively lengthy period of time.
Aluminium material is generally highly resistant to corrosion in a neutral environment because of the fact that the aluminium surface is oxidised and the thus formed oxide layer is relatively corrosion-resistant. In acidic (pH&lt;4) and alkaline (pH&gt;9) environments, this oxide layer becomes, however, unstable and so the material corrodes.
In order to achieve improved corrosion resistance, machine parts and structures for use in acidic or alkaline environments can be surface treated by means of coating with a suitable chemical-resistant polymer possessing superior internal strength and adhesion to the surface of the aluminium object, such as, for example, polymers containing fluorine. Fluorine-containing polymers normally also possess superior thermal resistance, which is an advantage in many fields of practical application.
One particular field of application for such polymer-coated corrosion-resistant aluminium objects is machine pats in filling machines intended for the packing of liquid foods of the type which fills, forms and seals packages in the same machine. In the handling of foods, extremely high demands are placed on hygiene and cleanliness, these demands being satisfied in that those parts of the machines which are in direct content with the food are regularly cleaned (i.e. at least once a day) by means of efficient detergents or cleaning agents. Such cleaning agents often contain alkaline chemicals. In cleaning, it is inevitable that detergents and cleaning liquids splash and drop onto other parts of the machine. In particular, machine parts which are included in the sealing unit such as, for example, sealing jaws, are often located beneath the filler pipe and the conduits which lead to and from the filler unit, which, on cleaning of the filler unit, inevitably results in cleaning agent dripping down onto these machine parts.
Surface treatment of high-strength aluminium by means of polymer coating today is put into effect in that the finished, already precipitation hardened and ready-to-use aluminium object is coated with a layer of polymer and then heated to elevated temperature in order to sinter or melt the polymer coating fast to the aluminium surface. How high the temperature which is to be selected is a matter of discretion taking into account the properties of the polymer and the temperature resistance of the aluminium. The term sintering (also known as agglomeration) is taken to signify the physical process which takes place when more or less solid material particles bond or frit to one another by molecular diffusion in the surface layer and thus "migrate together" to form a continuous microporous network.
Commercially available polymer compositions with a high melting point and which are sintered at high temperatures such as, for example, approx. 400.degree. C. display generally better adhesion, mechanical properties and resistance to chemicals. Heating to such elevated temperatures entails, however, that the aluminium material loses both hardness and mechanical strength by more than 50% up to approx. 65-75%. In practice, polymers are therefore employed which melt and sinter at lower temperatures, such as, for example, at approx. 200.degree. C. Coatings of such polymers unfortunately display poorer adhesion to the aluminium surface and, as a result, afford a poorer corrosion protection, while, on the other hand, the hardness and mechanical strength of the aluminium object are retained on heating up to at most approx. 200.degree. C.
Even though such plastic-coated machine parts of high-strength aluminium today constitute the most corrosion-resistant alternatives on the market which also satisfy other design and construction requirements, they must be replaced after a relatively short service life because the polymer coating has been attacked and weakened by the alkaline substances and no longer affords blanket protection for the aluminium object which, as a result, will be destroyed by corrosion. It is, thus, an as yet unsolved problem within the prior art technology to surface-coat objects of high-strength aluminium in order to achieve improved corrosion resistance to a sufficiently high degree without negatively influencing the mechanical strength and durability properties of the aluminium object.